PLL technology, as a technology necessary for a grid-connected inverter to tracking a power grid voltage, varies depending on whether or not the inverter is a single-phase or a three-phase grid-connected inverter. There are mainly the following three phase locking ways for the single-phase grid-connected inverter: (1) acquiring information about a period and a phase by detecting a voltage zero-crossing point; (2) a PLL technology based on a multiplier; and (3) a phase-locked loop technology based on a three-phase phase-locked loop controlling mode.
For a basic concept of the three-phase PLL controlling mode, a three-phase power grid is created artificially, and then the calculation is performed in accordance with the three-phase PLL controlling mode. Based on the basic concept of the three-phase PLL controlling mode, two virtual phases are created, and then a three-phase signal is converted into a signal in a dq rotating coordinate system. In the related art, the methods for creating the two virtual phases in the three-phase PLL controlling mode include a method of delaying by a ¼ period, a differential method, and so on, and these methods may be used to track the power grid when the power grid is in stable state. However, when the power grid is in a non-ideal state, the phase locking ability will be insufficient.